1. Field of the Invention
The present invention relates to an electrostatically oscillated device, such as an electrostatically oscillated angular velocity sensor or an electrostatically oscillated actuator, which has an oscillator that is oscillated by electrostatic forces.
2. Description of Related Art
For instance, an electrostatically oscillated angular velocity sensor, which is fabricated by, for example, etching a semiconductor substrate to form a base, an oscillator and driving electrodes for driving the oscillator, has been proposed as the electrostatically oscillated device (see, for example, Japanese Unexamined Patent Publication No. 2003-511684, which corresponds to U.S. Pat. No. 6,470,748, or Japanese Unexamined Patent Publication No. 2001-91265, which corresponds to U.S. Pat. No. 6,450,033).
In the electrostatically oscillated angular velocity sensor, the oscillator is driven through the driving electrodes to generate drive oscillation in a predetermined direction. In this oscillated state, when an angular velocity is applied, the oscillator is also oscillated in an orthogonal direction, which is orthogonal to the predetermined direction of the drive oscillation, to generate measurement oscillation due to generation of a Coriolis force. Through measurement of this measurement oscillation in the orthogonal direction, the degree of the angular velocity is determined.
Specifically, in the electrostatically oscillated angular velocity sensor, the oscillator is oscillated by the electrostatic forces. More specifically, flat electrodes or toothed electrodes are provided on left and right sides, respectively, of the oscillator. A predetermined voltage is applied to the oscillator, and two voltages, which are of opposite phases, are applied to the left and right driving electrodes, so that the oscillator is oscillated.
However, in the previously proposed electrostatically oscillated angular velocity sensor, there is only a little drive force, which corresponds to a difference between an attractive force of the left driving electrode and an attractive force of the right driving electrode. Thus, the efficiency of the sensor is not high. This point will be more specifically described with reference to FIG. 10.
FIG. 10 is a schematic plan view showing a structure of one previously proposed electrostatically oscillated angular velocity sensor, i.e., one previously proposed electrostatically oscillated device.
In the manufacturing of such an angular velocity sensor, a silicon-on-insulator (SOI) board, which includes two silicon plates that are joined through an oxide film, is processed using known semiconductor processing technology.
An oscillator 30 is secured to a base 20 through driving bridges 33. The driving bridges 33 are resiliently deformable in an x-direction in FIG. 10. Toothed driving electrodes 40, 41 are secured to the base 20. The driving electrodes 40, 41 apply electrostatic forces to the oscillator 30 to drive the oscillator 30 and thereby to generate the drive oscillation of the oscillator 30 in the x-direction. The driving electrodes 40, 41 include first and second driving electrodes 40, 41. In FIG. 10, the first and second driving electrodes 40, 41 are provided on the left and right sides, respectively, of the oscillator 30 to oppose each other in the x-direction.
Furthermore, in FIG. 10, a sensing mass 32 is arranged in the center of the oscillator 30. The sensing mass 32 is connected to the rest of the oscillator 30 by sensing bridges 34, which are resiliently deformable in a y-direction. Two sensing electrodes 50 are secured to the base 20 at two locations, respectively, which are opposed to the sensing mass 32.
In the electrostatically oscillated angular velocity sensor shown in FIG. 10, a predetermined voltage is applied to the oscillator 30, and two alternating voltages (drive signals), which are of opposite phases, are applied to the left and right driving electrodes 40, 41, respectively. As a result, the oscillator 30 is driven through the driving bridges 33 to generate the drive oscillation of the oscillator 30 in the x-direction.
Specifically, the predetermined voltage V0 is applied from a direct current (DC) power source 110 to the oscillator 30. Through use of an alternating current (AC) power source 100 and an inverter 120, the alternating voltage V1 is applied to the first driving electrode 40, and the alternating voltage V1′, which has the phase that is opposite to the phase of the alternating voltage V1, is applied to the second driving electrode 41.
Thus, the electrostatic attractive force F1 is exerted between the first driving electrode 40 and the oscillator 30, and the electrostatic attractive force F2 is exerted between the second driving electrode 41 and the oscillator 30. The electrostatic force F1, which is exerted between the first driving electrode 40 and the oscillator 30, is expressed by F1∝|V0−V1|. Similarly, the electrostatic force F2, which is exerted between the second driving electrode 41 and the oscillator 30, is expressed by F2∝|V0−V2|.
A difference (F1−F2) between the electrostatic attractive force F1 and the electrostatic attractive force F2 is used as a drive force for generating the drive oscillation of the oscillator 30 in the x-direction.
In the above state where the oscillator 30 is driven to generate the drive oscillation, when an angular velocity Ω is applied around a z-axis in FIG. 10, the Coriolis force is generated in the oscillator 30 in the y-direction. Thus, the sensing mass 32 of the oscillator 30, which is supported by the sensing bridges 34, is oscillated in the y-direction by the Coriolis force to produce measurement oscillation.
The capacitance between each sensing electrode 50 and the sensing mass 32 changes due to the measurement oscillation. The change in the capacitance is measured through the corresponding C/V converter 130 to determine the degree of the angular velocity Ω.
When the oscillator 30 is driven in the above described manner to generate the drive oscillation, the angular velocity Ω can be measured. However, as described above, only the small difference (F1−F2) between the electrostatic attractive forces F1, F2 is used to drive the oscillator 30. Thus, the efficiency of the drive oscillation is not high.
The above disadvantage is not specific to the electrostatically oscillated angular velocity sensor but is commonly encountered when the efficiency of the drive oscillation, i.e., the amplitude of the drive oscillation needs to be increased in the electrostatically oscillated devices, which has the above oscillator and driving electrodes.
Furthermore, the Coriolis force is proportional to the oscillating speed of the drive oscillation of the oscillator 30. Thus, the oscillating speed of the drive oscillation of the oscillator 30 needs to be increased to increase the sensitivity of the angular velocity and thereby to accurately sense the angular velocity. To achieve this goal, the number of the electrode teeth of each driving electrode needs to be increased to increase the drive force, i.e., the electrostatic force. For example, in the case of the sensor shown in FIG. 10, the number of the electrode teeth of each driving electrode 40, 41 should be increased.
However, when the number of the electrode teeth of each driving electrode is simply increased, the size of the board of the sensor is disadvantageously increased. To address this disadvantage, the inventor of the present invention has made a prototype electrostatically oscillated angular velocity sensor.
The prototype sensor of FIG. 11 is produced by forming two frames 131 in the left and right parts, respectively, of the oscillator 30 of the prototype sensor of FIG. 10.
In FIG. 11, a portion of each driving electrode 140, 141, which is secured to a base 320, is placed inside the corresponding frame 131 to form a frame interior side securing portion 160. A toothed driving electrode portion 140b, 141b is provided to each frame interior side securing portion 160, which is surrounded by the corresponding frame 131.
That is, each driving electrode 140, 141 of the second prototype sensor has the toothed first driving electrode part 140a, 141a and the toothed second driving electrode part 140b, 141b. The first driving electrode part 140a, 141a is opposed to the corresponding outer side of the oscillator 330, and the second driving electrode part 140b, 141b is provided to the corresponding frame interior side securing portion 160 and is opposed to the inner side of the corresponding frame 131.
When the oscillator 330 is formed to have the frames 131, and the toothed driving electrode part 140b, 141b is additionally provided inside each frame 131, it is possible to increase the number of the electrode teeth to increase the total effective electrode surface area, which aids in the drive oscillation of the oscillator 330. Due to the increase of the number of the electrode teeth, which is made possible by the additional second driving electrode parts 140b, 141b, the electrostatic force, which is applied to the oscillator 330, should be increased.
However, in the case of the structure shown in FIG. 11, an opposite electrostatic force, which is exerted in a direction opposite from the driving electrostatic force used in generation of the drive oscillation of the oscillator 330, is exerted in each space (hereinafter, referred to as a back surface side space) 170. The back surface side space 170 is defined between a back surface 160a of each frame interior side securing portion 160, which is opposite from the teeth of the corresponding second driving electrode part 140b, 141b of the frame interior side securing portion 160, and the inner side of the frame 131, which is opposed to the back surface 160a. 
Thus, although the number of the electrode teeth is increased, the drive force is not increased in proportional to the number of the electrode teeth, so that the oscillating speed of the drive oscillation of the oscillator 330 is not proportionally increased. The above disadvantage is not specific to the electrostatically oscillated angular velocity sensor but is commonly encountered when the drive force needs to be increased in the electrostatically oscillated devices having the oscillator that includes the frame, which surrounds the toothed electrode portion.